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Recent investigations by the International Decade for the East African Lakes (IDEAL) have significantly advanced our knowledge on the history of Lake Victoria. Seismic reflection profiles confirm the origin of the lake as a result of regional tilting and provide an estimated age of 400 000 years for the lake basin. Three major desiccation events are recorded in the seismic records that may reflect the 100 000 year Milankovitch cycle of climate forcing. The most recent arid period resulted in complete desiccation of the pre-existing lake. Lake Victoria arose from a dry landscape 14 600 calendar years ago (14.6 ka). Primary production was extremely high as lake level rose in its first 500 years, nourished by the high input of nutrients from the flooded landscape. A few species of cichlids and other fish swam out of their fluvial refugia to colonize the new lake, generating hundreds of new endemic species over the ensuing 14 000 years. Lake level rose until a brief overflow event at about 14.2 ka to 13.6 ka. Closed-basin conditions returned during the Younger Dryas until 11.2 ka, when major outflow and open-basin conditions were permanently established. The lake experienced progressively stronger stratification and water-column stability through the first half of the Holocene, and diatom productivity dropped to a minimum from 9.8 to about 7.5 ka. This period coincides with, but is much longer than, the 8.2 ka climatic event that has been observed in many Holocene records from throughout the world. The degree of water-column mixing appears to have steadily increased over the last 6000 years. Short cores from the open lake, document a shift in lake conditions beginning in the 1930s that progressed to the major ecosystem collapse of the early 1980s. The coincidence of the shift in sediment properties in the 1930s with the beginning of rapid expansion of human population and agricultural activity suggests cause and effect. It is conceivable that the lake experienced similar conditions due to natural causes between about 9800 and 7500 years ago.
Nine hydrological issues relating to land use and climate change are identified from a southern Africa perspective, each illustrated by an example based on field observations or simulation modelling. The nine issues are that (i) southern Africa's hydrological regime is already so variable that climate change will be difficult to detect; (ii) fluctuations in the hydrological regime are amplified by fluctuations in climate; (iii) hydrological responses are highly sensitive to land use changes; (iv) local scale abrupt land use changes may be hydrologically more significant than regional scale gradual changes; (v) land use change frequently exacerbates already variable flow regimes; (vi) detailed spatial information is vital in assessing impacts of critical land uses; (vii) major components of the hydrological system respond very differently to climate change; (viii) in developing countries inter-seasonal climate change may be more important than that at decadal time scale; and (ix) there is need to identify the hydrologically sensitive areas of a region.
Tropical regions, with their high biological activity, have the potential to emit large amounts of trace gases and aerosols to the atmosphere. This can take the form of trace gas fluxes from soils and vegetation, where gaseous species are produced and consumed by living organisms, or of smoke emissions from vegetation fires. In the last decade, considerable scientific effort has gone into quantifying these fluxes from the African continent. We find that both biogenic and pyrogenic emissions have a powerful impact on regional and global atmospheric chemistry, particularly on photooxidation processes and tropospheric ozone. The emissions of radiatively active gases and aerosols from the African continent are likely to have a significant climatic effect, but presently available data are not sufficient for reliable quantitative estimates of this effect.
Whereas zooplankton in most eastern boundary current systems (EBCSs) studied to date has declined in terms of biomass over the past 5 decades, it has increased 100-fold in terms of numerical abundance in the southern Benguela region, thereby undergoing a displacement in species composition through time. These long-term changes in coastal zooplankton off South Africa have previously been hypothesized to be mediated by both “bottom-up” and “top-down” control mechanisms, in contrast to other EBCSs and marine ecosystems in general which have traditionally been viewed to be structured from below. From a “bottom-up” perspective, the increase in zooplankton abundance is a long-term biological response to intensified coastal upwelling, common to EBCSs, and its associated processes of nutrient enrichment and increased phytoplankton production and biomass. On the other hand, such increase in zooplankton and the accompanying shift in its community structure are thought to result from a “cascading” trophic effect of predator-prey interactions between apex predators and their small pelagic forage species, and between the latter and their zooplanktonic prey. This study provides new information in terms of decadal-scale increasing trends in nutrient concentration and phytoplankton biomass off the West Coast of South Africa, lending support to the above “bottom-up” hypothesis. In addition, published data on long-term, density-dependent increases in pelagic fish condition and maturity are interpreted within the context of the above “top-down” mechanism which is postulated to control zooplankton populations in the Benguela region concurrently with “bottom-up” mechanisms.
The globalization of the agro-industrial food chain in Kenya can be traced back to the restructuring of the international division of labor. The current global agro-food industrial system entails aspects of a complex international division of labor, whereby the South specializes in high-value “non-traditional” exports, while the North concentrates on low-value cereal exports. In their contribution towards global change the relationships between agriculture and industry, and urban and rural areas are inextricably linked. The dynamics of these linkages form a crucial part of this process of globalization. This article examines the impact of the globalization of the vegetable commodity chain on the rural economy in Kenya. This has led to the establishment of multinationally-owned vegetable canning firms in Kenya which are geared towards the international markets, which in turn link up the rural economy in Kenya, with other global players through contract farming, and the manufacturing production chains. The implications of these processes on the rural economy, include environmental degradation, food insecurity, and aggravation of income inequalities among the Kenyan producers.
Regulation of rivers by damming as well as eutrophication in river basins has substantially reduced dissolved silicon (DSi) loads to the Black Sea and the Baltic Sea. Whereas removal of N and P in lakes and reservoirs can be compensated for by anthropogenic inputs in the drainage basins, no such compensation occurs for DSi. The resulting changes in the nutrient composition (DSi:N:P ratio) of river discharges seem to be responsible for dramatic shifts in phytoplankton species composition in the Black Sea. In the Baltic Sea, DSi concentrations and the DSi:N ratio have been decreasing since the end of the 1960s, and there are indications that the proportion of diatoms in the spring bloom has decreased while flagellates have increased. The effects on coastal biogeochemical cycles and food web structure observed in the Black Sea and the Baltic Sea may be far reaching, because it appears that the reductions in DSi delivery by rivers are probably occurring worldwide with the ever increasing construction of dams for flow regulation.
A frequent conclusion based on study of individual records from the so-called Medieval Warm Period (∼1000-1300 A.D.) is that the present warmth of the 20 th century is not unusual and therefore cannot be taken as an indication of forced climate change from greenhouse gas emissions. This conclusion is not supported by published composites of Northern Hemisphere climate change, but the conclusions of such syntheses are often either ignored or challenged. In this paper, we revisit the controversy by incorporating additional time series not used in earlier hemispheric compilations. Another difference is that the present reconstruction uses records that are only 900–1000 years long, thereby, avoiding the potential problem of uncertainties introduced by using different numbers of records at different times. Despite clear evidence for Medieval warmth greater than present in some individual records, the new hemispheric composite supports the principal conclusion of earlier hemispheric reconstructions and, furthermore, indicates that maximum Medieval warmth was restricted to two-three 20–30 year intervals, with composite values during these times being only comparable to the mid-20 th century warm time interval. Failure to substantiate hemispheric warmth greater than the present consistently occurs in composites because there are significant offsets in timing of warmth in different regions; ignoring these offsets can lead to serious errors concerning inferences about the magnitude of Medieval warmth and its relevance to interpretation of late 20 th century warming.